NASA's program to build a heavy-lift rocket capable of launching astronauts into deep space is moving from the design to the development phase.

The Space Launch System, which is designed to have an unprecedented lift capability of 143 tons, is scheduled to make its initial launch no later than November 2018.

"We are on a journey of scientific and human exploration that leads to Mars," NASA Administrator Charles Bolden said in a statement. "And we're firmly committed to building the launch vehicle and other supporting systems that will take us on that journey."

After retiring the aging fleet of space shuttles, NASA decided to rely on commercial spacecraft to act as space taxis, ferrying supplies, and eventually astronauts, back and forth to the International Space Station.

And by shrugging off the burden of building and maintaining lower-Earth-orbit spacecraft, NASA focused on building robotics and the heavy-lift rocket, both of which will be needed to extend the agency's exploration of deep space.

On Wednesday, NASA officials announced a major milestone in developing that heavy-lift system, which has a development budget of budget of more than $7 billion.

William Gerstenmaier, NASA associate administrator for the Human Exploration and Operations Mission Directorate, said during a conference call Wednesday that the agency has been working on the Space Launch System for three years and is making great progress.

At this point, NASA has robotic welding equipment ready to begin putting the pieces of the rocket together. It also has done some structural modeling and testing to see how the solid rocket boosters for the system will act, while also putting together a large vertical assembly center.

"This isn't about a single flight," said Gerstenmaier. "We need a system that is producible and affordable to fly on more than a single basis. We can't just focus on one launch. We're working to move humans deep into the solar system as soon as we can."

For its first test flight, NASA plans to have the rocket use just 70 tons of its 143-ton lift capability and carry an uncrewed Orion spacecraft beyond low-Earth orbit.

"Our nation is embarked on an ambitious space exploration program, and we owe it to the American taxpayers to get it right," said Robert Lightfoot, NASA associate administrator. "After rigorous review, we're committing today to a funding level and readiness date that will keep us on track to sending humans to Mars in the 2030s -- and we're going to stand behind that commitment."